Mobile multimedia broadcast is a broadcast technology coming into recent use. Using this technology, multimedia services like television can be provided on the handheld terminal even if it is moving fast. The terminal receives a multimedia service program list through a wireless broadcast channel, and can select any one from among authorized television channels, so as to receive multimedia data of the selected channel for watching television on the mobile terminal.
The encryption and authorization system of a mobile multimedia broadcast system encrypts the media data such as audio and video data, so that only authorized users can decrypt the media data with the encrypted control message, and can then play the programs of multimedia broadcast normally, while unauthorized users cannot decrypt the media data with the encrypted control message, which guarantees the normal operation of mobile multimedia broadcast.
During the above-mentioned process, the mobile multimedia broadcast system needs to transmit the encrypted control message to a user through an appropriate transmission channel, and the user obtains the encryption key from the encrypted control message to decrypt media data, thereby accessing multimedia broadcast services normally. There are many methods for transmitting an encrypted control message, among which the traditional out-band transmission method can transmit an encrypted control message through a transmission channel of a mobile communication network or that of a wideband wireless network, but correlation indications of services have to be added, so as to indicate that the transmitted encrypted control message will be used for a certain specified multimedia service or a certain type of established multimedia services, which will lead to the increase in use of transmission bandwidth and further to the increase of operation cost and finally will go against the development of broadcast network. The in-band transmission method uses the same mobile multimedia broadcast channel as the service media data to transmit the encrypted control message, for example, a digital television broadcast system packs an encrypted control message into transmission stream packets for transmission, because it also requires adding correlation indications of services to use transmission packets of separated service media data, and this will also consume more bandwidth.
It can be seen from the above-mentioned description that, traditionally, no matter which method is adopted to transmit an encrypted control message, correlation indications of services have to be added, which will occupy some transmission bandwidth, thus will reduce the utilization ratio of transmission bandwidth, and thus will further increase the operation cost of a mobile multimedia broadcast network. Therefore, a method for transmitting an encrypted control message that can overcome or avoid the above-mentioned disadvantages is required.